Conventionally, methods have been widely utilized which produce high-strength and high-elastic-modulus polyolefins by orientating molecules of polyolefins as by drawing or spinning. It is also widely known that orientation of polyolefin by drawing readily results in the improvement of its strength and elastic modulus, if the polyolefin is polyethylene which has a high theoretical elastic modulus.
Since drawing of a material with a smaller sectional area is easier, spin drawing has been primarily selected when a target draw ratio is high, e.g., 15 or higher. Meanwhile, Japanese Patent Laying-Open No. Sho 52-77180 discloses that an oriented tape can be obtained by a sequence of roll-drawing and drawing. However, even this method has encountered the extreme difficulty in producing an oriented sheet with a width of 50 mm or greater and suffered from the one or more of the following problems: cracking occurs in the sheet during a drawing process; necking accompanied by width or thickness variation occurs; fracture of the sheet occurs.
High-density polyethylene having little side chains, among polyethylenes, can be stretched at a high ratio because of the increased occurrence of intermolecular slippage and thus can be readily increased in strength and elastic modulus by drawing. However, such polyethylene exhibits poor creep resistance due to the intermolecular slippage that is maintained at a high degree of occurrence even after it has been oriented by drawing, which has been a problem.
As measures to increase the creep resistance, Japanese Patent Laying-Open Nos. Hei 2-296756 and Hei 6-280111 disclose methods wherein an ultra-high molecular weight ethylene/α-olefin copolymer having moderate side chains is diluted with solvents, various types of waxes or the like and then gel spun.
However, the high price of the ultra-high molecular weight ethylene/α-olefin copolymer adds to the production cost of resulting oriented polyolefin sheets. In addition, the gel spinning technique has a difficulty in imparting a shape other than the needle-like shape. Further, the technique needs an additional process for solvent removal. This inevitably reduces productivity and increases a production cost.
In the attempt to increase productivity, Japanese Patent Laying-Open No. Hei 2-258237 discloses a solvent-free method in which ultra-high molecular weight polyethylene is pressed, roll-drawn and stretched to produce an oriented polyethylene tape. However, this prior art has been insufficient to obtain an oriented tape with superior creep resistance.
U.S. Pat. No. 5,505,900 discloses a production method which was focused on the productivity and creep resistance improvements. In this method, a resin composition containing polyethylene and a photoinitiator is oriented while it is crosslinked by exposure to an ultraviolet radiation. However, the orientation process disclosed in U.S. Pat. No. 5,505,900 is a uniaxial orientation process without a roll-drawing treatment and thus difficult to achieve high-ratio orientation. In addition, the resin composition used shows declining light transmittance as crosslinking proceeds, making it difficult to achieve effective ultraviolet crosslinking. This has been another problem.